The present invention relates to a method of forming polymeric particles for use as electrophotographic toners and, more particularly, to a method of controlling particle shape, particle size, and particle size distribution of the toners for high-resolution electrophotographic image development.
In the conventional method of making electrophotographic toner, polymer resin binder and various additives such as colorants, charge control agents, release agents are melt-blended on a hot press roller or in an extruder. The resin mixture is then cooled and solidified, and then ground or pulverized into particles.
The above conventional process has numerous drawbacks including a desired resin brittleness required for pulverization into particles. Low molecular weight resins are typically used to obtain the desired brittleness. Low molecular weight is undesirable, however, due to regarding the following characteristics:                a. low molecular weight resins tend to be in the form of flakes while in use as toners or developers;        b. toner particles made of low molecular weight resins are prone to result in deposition on carriers, which cause poisoning of carrier particles in electrophotographic development;        c. low molecular weight resins have insufficient viscoelastic properties, such that hot-offset is easily formed due to toner sticking to the fuser roller during melting; and        d. thermal properties such as glass transition temperatures of the low molecular weight resins are hard to adjust, control, and keep in the desirable range.        
In addition, the conventional method further has the following common problems:                a. pulverization results in a wide distribution of particle sizes, such that the yield of useful toners is low and therefore the manufacturing cost is excessively high;        b. surface features of the particles are coarse and irregular, causing poor printing quality; and        c. particle size is hard to control, and many fine particles are produced by pulverization, causing a shortened service life of the developer due to accumulation of the fine particles on the developer.        
To resolve the above-mentioned problems and drawbacks, numerous toner manufacturing processes have been proposed and taught in the art. Many of the improved methods are based on aggregation and coalescence of particles in the solution.
Such processes overcome the drawbacks of the conventional melt-blending and pulverization process and have the following advantages:                a. coalescence can be controlled to obtain uniform shapes, and narrow particle size distribution without forming fine particles;        b. tough resins can be used by the solution coalescence process and thus the choice of usable resins is more accommodating than the conventional method; and        c. toner additives not applicable in the conventional melt blending method due to their decomposition by heat can then be used in the coalescence process.        
Typical coalescence processes employ dispersion of polymer solution in an aqueous phase that contains anti-coagulant agent to form fine resin droplets, adjusting the extent of aggregation and coalescence between resin droplets so as to control the sizes and the size distribution for the polymer droplets. The solvent of the polymer droplets is then removed to form solid polymer particles. The process is continued by separation, washing and drying to obtain a narrower distribution of particle sizes compared to the conventional pulverization process.
Three different coalescence processes are disclosed in the prior art relating to particle formation in the solution.
The first method includes using monomers to polymerize and form polymer particles by suspension polymerization or emulsion polymerization in which the dispersion and coalescence techniques are disclosed such as those in U.S. Pat. Nos. 3,959,153, 4,816,366, 5,418,108, 5,702,860, 5,968,705, 6,033,822, 6,190,819, 6,458,502, 6,503,677 and 6,596,453.
This method also has limitations. Special monomers and reaction initiators are required for suspension polymerization and emulsion polymerization. Therefore, the available polymer materials are limited. Currently, this method is only suitable for the styrene-acrylic copolymer system. The polymerization reaction is easily interfered with by other additives such that the degree of polymerization is limited. Unreacted monomers due to the low degree of polymerization can pose serious concerns in safety while handling. This complicates the manufacturing process and special care must be taken to remove the unreacted monomers.
The second method employs heating and melting the polymers instead of using solutions. The polymer resins are liquidized at high temperatures, then dispersed and coalesced in non-solvents as disclosed in U.S. Pat. Nos. 5,609,979, 6,287,742, 6,531,255 and 6,582,867.
In this method, the polymer resins having high melting points are difficult to operate and consume an excessive amount of energy. For resins with melting points higher than 100° C., water cannot be used as the dispersion medium, thus complicating manufacture. Further, waste management costs are increased. In addition, toner additives with insufficient thermal stabilities cannot be used and therefore the applications are limited.
The third method, as disclosed in U.S. Pat. Nos. 4,833,060, 4,835,084, 5,049,469, 5,283,149, 5,298,355, 5,968,702, 6,156,473, 6,294,595, 6,403,274, 6,482,562 and 6,682,866, are related to a solution coalescence process in which a polymer solution is prepared by dissolving a polymer in a solvent.
By this method, a wide range of polymer materials can be selected and the manufacturing process can be simplified, e.g. no monomer residues resulted and no heating at high temperatures is required. This method, however, has the following disadvantages.
First, to control particle size and particle size distribution, a high-speed homogenizer is used to form fine resin droplets and to perform coalescence.
Second, the solvent of the polymer solution has to be removed by evaporation after particle coalescence. Rapid evaporation may disturb the state of coalescence and unexpectedly change the size of the solidified droplets. Thus, evaporation of solvent is a slow, time-consuming process. This prolonged heating and evaporation creates a bottleneck in scaling up the manufacturing process.
Third, during the heating and stirring process, the dispersion stability is easily destroyed, causing agglomeration of coalesced droplets. As a result, the enlarged particles have a very wide distribution and the toner quality cannot be precisely controlled.
To resolve the drawbacks of the third method, U.S. Pat. No. 5,580,692 discloses a solvent extraction method to remove the solvent of coalesced droplets. The solvent extraction method adds a second solvent to extract the solvent contained in the coalesced droplets. Therefore, the organic solvent can be removed without heating. However, the state of solution dispersion may be unstable due to the addition of the second solvent. In addition, the amount of second solvent required for extraction may be excessively voluminous and create problems in waste management.
Based on the above analysis, the third method seems to provide the most advantages in preparing the polymer solution since there is no limitation in material selection. However, this method still has several limitations requiring improvement.
First, to reduce the equipment costs and power consumption, replacing of high-speed, large power-driven homogenizers with common mechanical mixers are desirable.
Second, the efficiency of removing the solvent from the coalesced droplets has to be enhanced without affecting the dispersion stability of the coalesced droplets.
Third, the particle size of the toner must be controlled more precisely to meet a variety of product requirements.
If the fine resin droplets cannot be formed through sufficiently small and stable dispersion, the expected size and size distribution of the particles after coalescence cannot be achieved. In addition, if the solvent cannot be removed in time, the coalesced droplets that have not been solidified will have many agglomerates during subsequent washing and filtering.
As the particle shape, particle size and particle size distribution of toner are all critical to printing quality, particularly for image development requiring high quality gray scales and full color. Particularly, the shape of the toner affects triboelectricity, flowability, cleaning efficiency, packing density and uniformity. Coarse toners such as those prepared by the traditional process have sufficient surface frictions, resulting in high triboelectricity and efficient blade cleaning. However, flowability, packing density and uniformity are inferior in coarse toners. The toner particles with uniform shape and narrow particle size distribution typically have uniform tribocharging and conforming toner stacking to result in high toner transfer efficiency, high print density and excellent print image quality. However, on the other hand, the cleanability and extent of tribocharging become less satisfied if uniformity of toner shape is approached. Different toner sizes result in different triboelectricity values during image development. Small-size toners are easily over-tribocharged, such that the toner transfer for image development becomes insufficient. In addition, non-uniform charging of toner particles may cause printing smear edges or backgrounding. Non-uniform sizes and irregular shapes result in loose stacking of toner layers in image development, thus thicker toner layers are required to give appreciable print density. For toners with regular sizes and shape, the stacking efficiency, image uniformity and color density are better, and thus the required thickness of toner layers is less. Thin toner layers and color uniformity result in better image transparency. Therefore, the print quality can reach a level close to that of offset printing. Conversely, thicker toner layers generate rough image surface and the printed paper is easily curled. Moreover, thinner toner layers are also advantageous in improving fixing efficiencies, such that the power consumption can be reduced and printing speed can be increased.
In brief, the aforementioned coalescence process revealed in the prior art have difficulties in controlling stable dispersion of the polymer colloidal solution. Therefore, the above methods for producing electrophotographic toners are limited by the problem of unsatisfactory control of the toner quality. A new method is thus required to produce uniform and high quality electrophotographic toners, so as to improve the efficiency of coalescence processes, and to provide better adjustment and control of particle shape, particle sizes and size distribution.